The discovery of frequent somatic mutations of epigenetic regulators in human cancers has highlighted a central role of epigenetic dysregulation in cancer pathogenesis. However, functions and mechanisms of action of these mutations remain largely unclear. The histone methyltransferase MLL3 (also known as KMT2C) is the most frequently mutated epigenetic regulator in breast cancer and also often altered in other major carcinomas. The majority of MLL3 mutations in breast cancer are gene deletion or protein-truncating point mutations. In addition, MLL3 expression is downregulated in breast cancers compared to normal breast tissues, suggesting it has a tumor suppressive role. Recent functional studies by us and others indeed have shown that MLL3 is an important tumor suppressor in different cancers. However, the mechanisms by which MLL3 mutations drive cancer pathogenesis remain poorly understood. Using novel tumor models that employ mammary stem cell (MaSC) and CRISPR technologies, we have shown that MLL3 deletion causes stem cell expansion and drives mammary tumorigenesis in cooperation with PI3K mutations, which significantly co-occur with MLL3 mutations in breast cancer. Our preliminary data also found that MLL3 mutation disrupted mammary epithelium homeostasis and induced multipotent stem cell activities in committed epithelial cell. Furthermore, we found MLL3 deletion sensitized cells to undergo epithelial- mesenchymal transition (EMT) in responding to various stimuli. These data suggest a role of MLL3 mutations in promoting cell plasticity in cancer development. EMT and related cell plasticity plays important role in cancer progression / metastasis and in therapeutic resistance. Consistently, we found MLL3 deletion greatly promoted tumor metastasis and resistance to endocrine therapy and PI3K inhibition in vivo. Based on these novel findings, we hypothesize that MLL3 mutation induces epithelial cell plasticity to promote tumorigenesis and metastasis and to confer resistance to cancer therapy. We will determine the role of MLL3 in regulating mammary epithelial cell plasticity during tissue homeostasis and oncogenic transformation (Aim 1), distinguish the role of MLL3 methyltransferase and adaptor functions in EMT and tumorigenesis (Aim 2), and determine the role of cell plasticity in MLL3 mutation-caused therapeutic resistance (Aim 3).